Ion selective electrodes (ISE) are used in the medical field, for example as blood analyzer assays. The particular ISE which the present invention is primarily concerned with is a multi-layer ISE having vacuum deposited Ag/Ni coatings deposited on a plastic substrate. U.S. Pat. No. 4,214,968, the disclosure of which is hereby incorporated by reference, discloses such ISE's and methods for their manufacture. A particular preferred substrate for the present invention is polyethylene terephthalate (PET).
Prior to the process of the present invention, the method of manufacturing Ag/Ni electrodes consisted of first providing a PET substrate with an adhesion promoting sub-coating thereon, such as poly(acrylonitrite-co-vinylidene chloride-co-acrylic acid), to improve the adhesion between the PET substrate and the deposited metal layers. A silver layer was then vacuum deposited over the sub-coating. Prior to the subsequent step of depositing nickel, the silver layer required aging of approximately 10 weeks. After the aging process, nickel was selectively deposited in a striped formation over the silver layer. The exposed silver areas were then exposed to a bleaching agent which converted a portion of the silver layer to AgCl. If the silver was not aged sufficiently prior to the Ni deposition step, then during the bleaching operation cracking would frequently occur in the silver layer between the nickel stripes, rendering the electrode useless. The mechanism which caused the cracking is unknown. Since the cracking occurred regardless of whether the adhesion promoting sublayer was employed and in fact occurred even on electrodes that exhibited relatively good PET/Ag adhesion, the cracking did not seem to be adhesion related. Consequently, prior to the nickel deposition, to achieve the requisite aging, the silver deposited on the PET substrate required storage for some 10 to 12 weeks, greatly increasing manufacturing cost. Further, prior to the present invention, an adhesion promoting sublayer, mentioned above, was needed to prevent both cracking and delamination of the electrode during manufacture.
Glow discharge pretreatment and similar techniques are known techniques for improving the adhesion between polymer substrates and metal coatings deposited by physical deposition processes. However, there is no teaching that such a pretreatment in a nitrogen atmosphere will prevent cracking of the silver layer in a Ag/Ni electrode, even though the nickel layer is applied without substantial prior aging of the silver layer.
High energy ion beams have been used to enhance the adhesion between deposited metal layers and organic substrates. In ion beam enhanced adhesion, a high energy beam of ions is incident upon the organic substrate prior to deposition of a metal layer. For example, copper can be deposited on polymer substrates by electron beam deposition, after high energy ion irradiation of the substrates. In this procedure, both reactive and inactive ions can be used, and the ion beam energy ranges upward from about 200 keV.
Such systems are costly and require complicated apparatus to produce the high energy, focused ion beam. The nature of the ion beam makes it impossible to be able to process large substrate areas simultaneously, and therefore this technique is not readily suitable for high throughput in commercial systems. Further, high energy ion beams can cause damage to the organic substrate and require large amounts of power. It is also extremely difficult to make large, high energy ion beams with the proper flux and power over a large area.
Glow discharge or plasma discharge has also been used to affect the polymer surface prior to metal deposition. In glow discharge techniques, a brush or glow discharge is created by introducing residual gasses into a vacuum chamber and applying a high voltage in order to create ions which activate the polymer substrate surface. Examples of the gasses which can be introduced into the vacuum chamber include air, oxygen, nitrogen, helium, neon, argon, krypton, xenon, and radon. After the glow discharge activation, the metal film is deposited by techniques such as electroless plating, evaporation, and sputtering.
A method for producing Ag/Ni ion selective electrodes is needed which does not require long aging periods for the Ag layer prior to the Ni deposition and bleaching operation. Ideally, the process should be one in which the Ag and Ni layers can be deposited within a short period of time, and ideally where they can be deposited during one manufacturing operation, after which the PET/Ag/Ni apparatus can be immediately bleached.